Be more consistent about object ids used in ref operation validation
In CreateBranch, a RefUpdate is created with expectedOldObjectId set
but not oldObjectId. In DeleteBranch[es], none of expectedOldObjectId,
oldObjectId, and newObjectId is set.
As a result, when RefOperationValidators uses oldObjectId and
newObjectId to create a ReceiveCommand, they can be null, resulting in
NullPointerException when a listener dereferences them.
So use expectedOldObjectId instead of oldObjectId in
RefOperationValidators. Before a ref update has been performed, only
the former is meant to be set.
In DeleteBranch[es], set the expectedOldObjectId and the newObjectId
(which is zero). This way, in RefOperationValidators we know both values
will always be set.
Bug: Issue 5817
Helped-By: Jonathan Nieder <jrn@google.com>
Change-Id: If1f3a6179fa789077731a16e4b731227a73be7f2

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